The semiconductor industry continually seeks new ways to create more sophisticated circuitry while using less surface area of a semiconductor substrate. To achieve this result, individual circuit elements of a semiconductor device may be fabricated closer together. The minimum space between individual circuit elements may be limited by the degree of contrast that may be produced by lithographic processes.
Lithography generally involves placing a radiation sensitive material (sometimes referred to as "photoresist" or "resist") on an oxide film. Typically, the resist is radiated through a mask placed over the resist by lithography techniques such as photolithography, electron beam lithography or x-ray lithography. The mask may be a piece of glass having chrome or another appropriate metal deposited on the mask in a predetermined pattern. The glass area between adjacent chrome areas may be referred to as "slits". The resist reacts chemically in the exposed areas of the mask depending upon the choice of the chemical system. The semiconductor substrate may thereby be further processed in the areas exposed by such lithographic techniques.
The width of slits of a mask may be decreased to allow fabrication of smaller individual circuit elements. Additionally, the width of the slits of a mask may be decreased to fabricate individual circuit elements closer together. Narrower slits allow less ultraviolet light to be transmitted through the mask toward the resist. As the width of a slit approaches the wavelength of the incident ultraviolet light, the ultraviolet light may cease to be transmitted through the slit. With narrower slits, it may be necessary to expose the resist for longer periods of time in order to adequately expose the resist. One particular problem with prior systems is that narrower slits of the mask result in low contrast on the resist layer.